Contemporary selective call receiver communication systems employ messaging schemes that can deliver voice, numeric, or alphanumeric messages to its user. The majority of selective call systems transmit address and message information using a protocol such as GSC (Motorola's Golay Sequential Code) or POCSAG (Great Britain's Post Office Code Standardisation Advisory Group).
When a user desires to transmit drawings, symbols or complex graphic-type information, existing selective call messaging (paging) systems and data transport protocols do not gracefully allow for the transmission of either long textual messages or messages containing graphical data. This is because selective call receivers are powered by limited energy content battery supplies that are quickly degraded when receiving and displaying long messages. In addition, because of the miniature size of the selective call receiver and its display, the information received is not easily readable and accessible to the user.
Thus, specialized techniques are necessary to transmit drawings and other more complex information to a selective call receiver. A facsimile message is an example of such complex information. However, transmitting a facsimile message to a selective call receiver has many advantages, one advantage being that the data file representative of the image is compressed, reducing the amount of air-time to transmit the message to a selective call receiver. For simplicity, the term "facsimile selective call receiver system" is used hereinafter to refer to a system for transmitting a facsimile to a selective call receiver.
Data compression in a facsimile selective call receiver (SCR) system involves computing the black and white run-lengths of the bi-level images followed by coding the run-lengths using the well known Huffman coding techniques. Huffman coding the run-lengths requires designing a Huffman code-book based on the estimated probability of the run-lengths. Huffman coding is optimal when the probabilities of the run-lengths are exact. However, the run-length probabilities vary for different types of images. Therefore, it is not possible to design a Huffman codebook which is close to optimum for different types of images with different resolutions.
Consequently, a more robust data compression and decompression system is needed in a facsimile SCR system to account for different types of images, while keeping the size of the compressed data file representative of the image as small as possible, ultimately to minimize air-time when wirelessly transmitting the facsimile message to a selective call receiver. Moreover, there is room for improvement in a facsimile SCR system, where the size of the display of a portable selective call receiver is presently small, and the size of the fax message image is also generally small.